Gain-equalizing filters, also known by the acronym GFF, which stands for “Gain Flattening Filter”, generally consist of Bragg gratings photo-written on portions of waveguides such as optical fibers or planar waveguides. A waveguide is conventionally composed of an optical core whose function is to transmit and possibly amplify an optical signal, surrounded by an optical cladding whose function is to confine the optical signal in the core. To this end, the refractive index, n1, of the core and the refractive index, n2, of the cladding are such that n1>n2. As is well known, the propagation of an optical signal in a single-mode waveguide breaks down into a fundamental mode guided in the core and secondary modes, also referred to as cladding modes, which are guided over a certain distance in the optical core/cladding assembly.
Either the core or the cladding of a waveguide can be “doped”, for example with germanium (Ge), so as to be made photosensitive for writing Bragg gratings within the waveguide. The gratings conventionally used for gain flattening applications are slanted Bragg gratings, known by the acronym SBG, or Long-Period Gratings, known by the acronym LPG. Such gratings do not reflect light wavelengths back into the core but are, instead, designed to allow coupling of the fundamental mode (of specific wavelengths) into the cladding modes. The use of such SBG or LPG gratings makes it possible to dispense with optical isolators, which are otherwise essential when the gain flattening is achieved with reflective gratings such as straight Bragg gratings.
Gain flattening filters are associated with optical amplifiers regularly distributed along transmission lines. As is well known, optical amplifiers do not generally provide equal amplification for all wavelengths of the signals transmitted over the various channels of the same transmission line. Since a signal carried on one wavelength (or channel) cannot be amplified preferentially to another signal carried on another wavelength, gain flattening filters are employed to keep the optical power constant within all channels. As is well known, a gain-flattening filter restores the power of all channels output from an optical amplifier to a common level by preferentially attenuating those channels that receive the greatest gain within the amplifier. For instance, a gain flattening filter (GFF) may be coupled to the output of an Erbium-Doped Fiber Amplifier (EDFA). Alternatively, a GFF may be incorporated into the EDFA itself within which it may be optically coupled to the signal output end of an Erbium-Doped Fiber or may be optically coupled between two lengths of Erbium-Doped Fiber. Since the gain function (gain vs. wavelength) of an optical amplifier can assume a complex shape, the transmission spectrum of gain flattening filters must be similarly complex and matched to the amplifier, in an inverse sense.
With the increase of bandwidth in WDM transmission and the multiplicity of possible designs according to properties of the transmission line specifications (length, power budget, etc.), and other intrinsic component dependent properties (e.g. gain, erbium response spectrum, etc.), the specifications and tolerances of the Gain Flattening Filters are stringent, particularly when considering the attenuation deviation error over the useful spectral range. It is extremely important to minimize any systematic deviations of the actual transmission spectrum from the actual shape required for perfect gain flattening, since such errors will accumulate over several spans of optical fiber line having several amplifiers. Thus, it is mandatory for the GFF spectral attenuation has to match the amplification curve of the EDFA within a close tolerance. Further, as optical amplifier technology evolves and the useful amplification spectral bandwidth increases, new attenuation functions, having even more complex spectral shapes and accentuated attenuation variations (i.e. steeper slopes) are required.
From a historical point of view, the realization of complex attenuating spectra in Gain Flattening filters was achieved, in the prior art, by concatenating a series (of a limited number, for instance, three) of specifically designed optical fibers, wherein one fiber Bragg grating filter was written into each fiber. Since each such fiber Bragg grating was devoted to attenuation within a particular spectral range, the overall attenuation function or properties were achieved through splicing or fusing several such FBG-bearing optical fibers parts together serially. This prior-art serial fiber gain flattening method was sufficient and sustainable as long as the spectral attenuation template was of a smooth classical shape—in the sense that no specific shape was required—that did not require adaptation to the needs of a particular application. Unfortunately, however, this serial fiber method suffers from poor sustainability and flexibility when shifting to more complex shapes, since, in such a case, development and selection of a new series of fiber Gratings, would require large expenditure of time.
To mitigate this drawback, it has been proposed to write the various slanted gratings on different portions of the same waveguide. Such a method is described in the patent application WO 93/24977. According to the contrast given to each grating, it is thus possible to model the response of the complex filter. Unfortunately, this prior-art requires as many writings through as many phase masks as there are elementary filters. The more complex the gain flattening profile, the higher the number of elementary SBG writings necessary for producing the complex flattening filter, which makes the manufacture of such filters more expensive.
To overcome these limitations, an improved apparatus and method was previously implemented in which a concatenation of several slanted Fiber Bragg Gratings is written into a single fiber through a single highly chirped phase mask (i.e., a diffraction grating whose spacing between grating rulings varies greatly along the mask along a direction perpendicular to the rulings) such that the whole gain flattening filter is comprised of the single fiber and, also, such that the value of the mean refractive index is constant throughout the fiber. This constant mean refractive index enables the minimization of the change of the spectral response at the annealing stage of the filter—a long-term stabilization of the device by application of a thermal treatment. Such a technique has been described in U.S. patent application Publication 2003-202745. A schematic illustration of a method for producing a gain-flattening filter according to this prior-art technique is shown in FIG. 1.
As shown in FIG. 1, the prior-art constant-mean-index SBG gain-flattening filter is manufactured by passing a UV beam 70-70a through a slit 60-60a and strongly chirped (typically 10 nm/cm) phase mask 50 onto a photosensitized fiber 38. The positions of the UV beam and/or the slit are moved relative to the phase mask 50 and fiber 38 using a translation support 80. Thus, in a first position, the UV beam 70 passes through the slit 60 so as to cause spatially filtered beam 72 to pass through portion 52 of the chirped phase mask 50 so as to generate elementary fiber Bragg grating filter 30.2 within the fiber 38. Subsequently, the positions of the UV beam and/or the slit are moved such that the UV beam, now represented as UV beam 70a, passes through the slit, represented as slit 60a, so as to cause spatially filtered beam 72a to pass through portion 52a of phase mask 50 so as to generate another elementary Bragg grating 30.3 within fiber 38. Other elementary fiber Bragg gratings, such as grating 30.1, etc., may be inscribed in the fiber 38 by irradiations at other positions. The grating rulings within the strongly chirped phase mask 50 are oriented such that each of the elementary fiber Bragg gratings 30.1, 30.2, 30.3, etc. is a slanted Bragg grating. The strongly chirped mask makes it possible to systematically change the spectral positions (i.e., the cladding-mode-coupled wavelengths) of the fiber Bragg gratings, relative to one another, along the length of the fiber. The whole useful spectral band is therefore easily covered over a reduced (less than about 30 mm for the C-band) fiber length.
In the prior-art constant-mean-index SBG gain-flattening filter technique (FIG. 1), the synthesis of the overall spectral shape of the gain flattening filter function is accomplished using elementary filter shapes that are equally spaced in wavelength. To fabricate each elementary filter 30.1, 30.2, 30.3, etc., the UV beam 72, 72a, etc. is positioned over a desired location 52, 52a, etc. on the strongly chirped mask 50 while the photosensitized fiber 38 is irradiated for a known time by the UV beam passing through said location on the phase mask. The position of the UV beam along the mask is then changed using a slit moving 60, 60a, etc. parallel to the length of the optical fiber 38. Because of the chirping of the phase mask 50, there is a straight correspondence between the UV beam position onto the strongly chirped masked and the spectral position (in wavelength) of the resulting Bragg grating written into the fiber.
In the prior-art constant-mean-index SBG gain-flattening filter technique (FIG. 1), the sequential position displacement of the UV-beam over the phase mask between consecutive positions, so as to produce consecutive elementary filters, is periodic (i.e. constant). This displacement, Δx, between successive UV beam positions, and, therefore, between each pair of adjacent filters of the n elementary filters 30.1-30.n, is equal to the UV-beam width (slit width) as shown in FIG. 2. This latter condition was imposed so that there would be no spatial overlap between two consecutive UV-irradiated regions onto the mask (FIG. 2). The distance ΔL is the length of each elementary fiber Bragg grating filter. As shown in FIG. 2, ΔL=Δx. Since there is a direct correspondence between the UV-beam position, relative to the mask, and the spectral filter position, in wavelength, each attenuating filter is equally spaced, in both wavelength and position with each neighboring filter. The spectral displacement, value, Δλ, between successive elementary filters is given by the relationship Δλ=neffΔL C, where neff is the effective refractive index of the guided mode and C is the chirp value of the phase mask (e.g. 10 nm/cm).
FIG. 3 is a graph 20 representing the synthesis of an attenuation curve 22 of a gain flattening filter, as produced by the prior-art single fiber SBG technique. The curve 22 is a summation of the transmission curves 24 of all the elementary fiber Bragg gratings 30.1, 30.2, 30.3, etc. The transmission curves 24 associated with the members of each pair of adjacent elementary Fiber Bragg Gratings are spaced Δλ apart. The strength or contrast (related to the depth of the curve on FIG. 3) of each elementary filter, in terms of its ability to couple the targeted wavelength, is produced by varying the efficiency of the interference pattern while the elementary filter is being written. This may be accomplished by, among other methods, vibrating the slit back and forth about its central position. As a result, although the strengths of the various elementary filters are generally different, the writing time (UV irradiation) is set to be equal for each elementary filter. Consequently, the value of mean refractive index value is constant for all such elementary filters, since this depends on the total UV irradiation time.
Unfortunately, the prior art constant-mean-index method, described above, of producing a single-fiber gain-flattening filter has a few disadvantages. The first problem is in the way the apodization of the elementary filters is carried out—the requirement to obtain a constant mean refractive index causes the writing time to be identical for all the elementary filters, regardless of the target contrast (even for weak contrast values). As a result, the total UV exposure time can be very long with this method, thereby causing inefficiency in the manufacturing process.
A second problem with the above-described prior art method is related to the deviation error, relative to the desired attenuation spectrum. FIG. 4 is a graph 25 of raw and smoothed error deviations observed for a gain flattening filter produced by the prior-art technique of FIG. 1. The raw deviation is shown as curve 26 and the mathematically smoothed version of this deviation is shown as curve 27 in graph 25 (FIG. 4), both plotted with respect to wavelength. The smoothed curve 27 provides an indication of the low-order or average trends of the error curve. However, by comparing the raw curve 26 with the smoothed curve 27, it may be observed that the error curve contains higher-order small scale oscillations that are superimposed upon these average trends. To better display these oscillations, curve 28 shows, on an expanded scale, the difference spectrum, which is obtained by subtracting curve 27 from curve 26.
The curve 28 of graph 25 (FIG. 4) clearly shows the systematic small spectral scale oscillations that are experimentally observed in the error deviation spectrum for a filter manufactured by the prior-art method of FIG. 1. These oscillations arise from the fact that the elementary fiber Bragg grating positions do not overlap with each other in the fiber comprising the gain flattening filter. These systematic and reproducible oscillations create a problem for long-haul transmission over fiber optic spans encompassing several cascaded optical amplifiers because the errors are cumulative.
Finally, the prior-art method of producing a single-fiber gain-flattening filter prior solution is not usable for the synthesis of new spectral attenuation shapes exhibiting large variations at both extremities of the covered spectral range. Large spectrally limited variations imply large derivative values and, thus, a bad match between the synthesized attenuation curve and the desired attenuation spectrum curve for filters made with constant spaced elementary filters.